1. Field of the Invention
This invention relates to a communication method, a transmitter and a receiver, and more particularly, is applicable to radio communication system such as a portable telephone system or the like.
2. Description of the Related Art
In this kind of radio communication system, an area for providing communication service is divided into cells having a desired size and a base station is set as a fixed radio station in the cell. The portable telephone device as a mobile radio station is installed in the cell so that the portable telephone device as a fixed radio station is constituted to be engaged in communication with the base station in the cell in which the portable telephone device is present. At that time, in this kind of radio communication system, the interference problem of an electronic wave is avoided by using a channel of frequency which is different between cells (this is simply referred to as a channel).
By the way, when a different channel is used between adjacent cells in this manner, the problem of interference can be avoided, but at the same time, there arises a disadvantage in that the usage efficiency of the frequency is reduced. As a consequence, in recent years, the interference problem is avoided by changing the channel which is used in each cell at random for each predetermined timing, and the usage efficiency of the frequency is improved.
By the way, in such radio communication system, the channel is only changed at random so that it is not necessarily possible to avoid that the channel becomes the same between adjacent cells. Thus, it is feared that the timing becomes the same. In the case where the same channel is used, there might arise an interference problem in the same manner as the prior art.
For example, as shown in FIG. 1, in the cell 1A, when the portable telephone device 2A uses the predetermined channel to be engaged in communication with the base station 3A, and the same channel as the cell 1A is used in the adjacent cell 1B so that the portable telephone device 2B is engaged in communication with the base station 3B, it sometimes happens that the sending signal CB which is sent by the portable telephone device 2B reaches the base station 3A. In this case, the sending signal CB which reaches the base station 3A becomes an interference wave (so-called hindrance wave) with respect to the sending signal CA which is sent to the base station 3A. In this manner, even when an interference wave I is generated to the sending signal CA in this manner, the signal level of the sending signal CA is larger than the interference wave I, and no considerable problem is generated in normal cases. However, in the case where because, for example, the portable telephone device 2A is distant from the base station 3A and the signal level of the sending signal CA becomes smaller than the interference wave I, the base station 3A receives the interference wave I by mistake instead of the sending signal CA from the portable telephone device 2A. As a consequence, the signal series transmitted by the portable telephone device 2B is restored, and the communication content of the other person is leaked with the result that a situation which is not preferable as a radio communication system might arise. For reference, in such a situation, even when the received data is not obtained as the radio communication system, it is desirable that the interference wave I is not demodulated.